1. Field of the Invention
The present disclosure is related to the disclosures provided in the following U.S. applications: "Intelligent System for Generating and Executing a Sheet Metal Bending Plan", filed in the names of David Alan Bourne et al. (application Ser. No. 08/386,369, which is a continuation of application Ser. No. 08/338,113); "Methods and Apparatuses for Backgaging and Sensor-Based Control of Bending Operations", filed in the names of David Alan Bourne et al. (application Ser. No. 08/338,115, now U.S. Pat. No. 5,835,684); and "Intelligent System for Generating and Executing a Sheet Metal Bending Plan", filed in the names of Richard M. Moore, Jr. et al. (application Ser. No. 08/385,829, now U.S. Pat. No. 5,761,940, which is a continuation of application Ser. No. 08/338,153); and the disclosures of all of these applications are expressly incorporated by reference herein in their entireties.
The present invention relates to a fingerpad force sensing system for providing electrical signals representative of the force of material in contact with a fingerpad force sensor. More particularly, the present invention relates to a fingerpad force sensing system which is used to provide an indication of the force applied to a planar object which is in contact with the fingerpad force sensor of the system.
2. Background and Material Information
In mass-production systems for fabricating products made from planar materials, such as, for example, sheet metal, there is a need for manufacturing the desired products quickly, accurately and at the lowest possible cost. Mass-production systems which produce large batches of a product are able to distribute the cost of errors and the set-up and fine tuning of the fabrication machine such that the cost per product produced is relatively low. However, there is a continuing emphasis on producing a product at the lowest possible cost.
The cost per product produced is even more important in small-batch and custom part manufacturing systems which do not have the economies of scale of the mass-production systems and, therefore, cannot spread the cost of errors and mistakes over the same large product batch sizes as the mass-production systems. Therefore, such small-batch and custom part manufacturing systems must utilize automated machines which manufacture the desired product correctly the first time in order to be cost effective and to produce the product at a competitive cost to other producers. Errors should be corrected either before or during production of the product. Nevertheless, it would still be desirable to be able to eliminate errors as much as possible when manufacturing a product, whether using a mass-production system with large batch sizes or a small-batch and custom part manufacturing system.
Typically, a majority of production errors in automated mass-production manufacturing systems occur because the manufacturing system is not able to adequately compensate for variations in the manufacturing process. One such variation, which will be discussed in connection with the example of an automated mass-production manufacturing system described herein, is the thickness of the sheet metal used in connection with the fabrication of sheet metal products. Since the manufacturing process variations are difficult to model before the actual manufacturing system is operational, sensors are used to detect and compensate for such manufacturing process variations in real time.
The fingerpad force sensor of the present invention can be used, in connection with, for example, an automated metal-bending work station that efficiently manufactures small-batch sheet metal parts described by computer aided design systems. The automated work station may include a process planner that selects the necessary punches, dies, grippers and sensors, determines the fabrication sequence and then generates the appropriate data for the software which operates the bending machine. After the process plan is formulated, a work station-based system provides real-time sensor-based control of the bending machine during the manufacturing process, while also recording the process history for later review by operators.
Using such an automated metal-bending machine without the fingerpad force sensor of the present invention creates several drawbacks. For example, since the original bending machine is programmed through teach-playback methods, a considerable amount of time is required in order to fine tune and adjust the bending machine to produce the desired part. In addition, even after the system has been fine-tuned, failures still occur during the manufacturing of parts. Such failures include, for example, collisions with the punch tools and poor bends due to part misalignment. Generally, such failures occur because the bending machine does not have the intelligence to accurately know the position and orientation of the workpiece.
There are many reasons for the part position uncertainty in prior art systems. They include the mechanical slop present in the loading mechanism for the bending machine, the loss of part position information when the robot gripper releases the workpiece during bending operations, slippage of the workpiece in the robot gripper during handling and flexing of the workpiece during handling. The present invention addresses such workpiece position uncertainty by augmenting the sheet metal bending machine system with fingerpad force sensors.
The fingerpad force sensor system of the present invention overcomes the above-described shortcomings of the art by detecting process variations which occur in, for example, the automated sheet metal bending manufacturing system described above. Several of those fingerpad force sensors are embedded in the gripping pads of the robot which forms part of the automated sheet metal bending manufacturing system. When external forces are applied to the sheet metal workpiece being held by the robot gripper, the deformation of the rubber pads produces a change in the outputs of the sensors. The sensors are designed as an integral part of the robot's gripper and therefore travel with the workpiece. The instant design thus allows the manufacturing system to monitor the "status" of the workpiece at any time during all phases of automated bending: material acquisition, material handling, machine loading and unloading. In each of these areas, there are problems of dynamic forces between the sheet metal and the gripper that must be actively sensed and controlled. In that manner, the sensors used with the present invention enable the manufacturing system to align the workpiece at the loading station and the press brake, to detect unplanned collisions between the workpiece and the manufacturing system and to also detect imminent workpiece slippage. Such imminent workpiece slippage in the robot gripper occurs when the robot accelerates large parts too quickly.